Add footer option

Apart from the banner option, it's useful to have a footer option
that can be used in order to wrap concatenated output in a block.

This is not the same case to this one: https://github.com/gruntjs/grunt-contrib-concat/pull/9/

In the previous version of grunt we used to have the same effect with an other trick that doesn't work now. Take a look here:
https://github.com/cbotsikas/Webinos-Platform/blob/master/grunt.js#L33
Lines: 33, 41, 72